This application claims priority to an application entitled xe2x80x9cApparatus and Method for Channel Estimation in Radio Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Dec. 29, 1999 and assigned Ser. No. 99-65273, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to data demodulation in a radio communication system, and in particular, to an apparatus and method for demodulating data against signal distortion caused by fading or other factors.
2. Description of the Related Art
Radio communication technology, mainly cellular communication technology has been rapidly developed and GMPCS (Global Mobile Personal Communication System) is being deployed for communication throughout the world. For radio communication systems utilizing satellites, research has actively been conducted on data demodulation because the long distance between a satellite and a mobile station causes severe data distortion due to fading.
In general, predetermined symbols (e.g., pilot symbols) are inserted in one frame prior to transmission and the distortion of other information symbols are compensated for by detecting the distortion of the predetermined symbols. That is, a transmitter inserts agreed symbols between data symbols prior to transmission and a receiver extracts those agreed symbols for use in channel estimation. Conventional channel estimation relies on the use of an interpolator or extended symbol-aided estimation (ESAE).
With respect to prior art systems, FIG. 1 is a schematic block diagram of a conventional channel estimating apparatus in a radio communication system for data recovery. FIG. 2 is the format of a frame used in the conventional radio communication system. FIG. 3 is a detailed block diagram of the conventional channel estimating apparatus in the radio communication system. FIG. 4 is a block diagram of another conventional channel estimating apparatus relying on ESAE in a radio communication system and FIG. 5 conceptually illustrates channel estimation for data recovery relying on the ESAE.
FIG. 1 illustrates a channel estimating apparatus using pilot symbols for channel estimation in a PSAM (Pilot Symbol Assisted Modulation) system. In the PSAM system, pilot symbols are periodically inserted by pilot symbol inserter 101 between data symbols and the entire signal is pulse-shaped by pulse shaper 102 prior to transmission. Fading and AWGN (Additive White Gaussian Noise) are added to the transmission signal by multiplier 103 and adder 104, respectively. A receiver separates an input signal into pilot symbols and data symbols by passing the input signal through a matched filter 105 and estimates the channel of data symbols using the pilot symbols. For the channel estimation, an interpolator 108 is required and data is recovered using the interpolation result. Delay 106 compensates for the signal delay through interpolator 108.
The channel estimation process in a general PSAM system can be expressed briefly as
St(t)=Re[zO(t)exp(j2xcfx80fct)]xe2x80x83xe2x80x83(1) 
zO(t)=zOi(t)+jzOq(t)xe2x80x83xe2x80x83(2) 
where St(t) is the transmitter signal, Re[zO(t)exp(j2xcfx80fct)] represents the real number part of zO(t)exp(j2xcfx80fct), fc is a carrier frequency, and zO(t) is a transmission baseband signal with its band limited by a transmission filter. As shown in FIG. 2, preset pilot symbols are inserted into a transmission frame. Due to fading, the transmission signal arrives at the receiver as
Sr(t)=Re[c(t)zO(t)exp{j2xcfx80(fcxe2x88x92fo)t}+nc(t)exp{j2xcfx80(fcxe2x88x92fo)t}]xe2x80x83xe2x80x83(3) 
where Sr(t) is the received signal, and nc(t) is the AWGN component. A channel complex gain c(t) includes fading and a frequency offset, given by
c(t)=exp(j2xcfx80fot)g(t)xe2x80x83xe2x80x83(4) 
where fo is a residual frequency offset and g(t) is the envelope of c(t). Then, a demodulated baseband signal is expressed as
U(t)=C(t)Z(t)+n(t)xe2x80x83xe2x80x83(5) 
It is necessary to estimate C(t) to achieve the baseband signal Z(t). The sampled value of an mth symbol in a kth frame is
tk, n={k+(n/M)}TPxe2x80x83xe2x80x83(6) 
for k=0, 1, 2, 3, . . . n=0, 1, 2, 3. . . Mxe2x88x921 
where TP, a pilot symbol insertion period, is NT. A pilot symbol demodulated at every frame timing instant is
U(tk, 0)=C(tk, 0)b+n(tk, 0)xe2x80x83xe2x80x83(7) 
An estimated value of fading at the instant when a kth pilot symbol is received is computed by dividing a distorted symbol U(tk, 0) of Eq. (7) by a pilot symbol b. That is,
C(tk, 0)=u(tk, 0)/b=C(tk, 0)+n(tk, 0)/bxe2x80x83xe2x80x83(8) 
Fading-caused distortion of an information symbol can be detected using an interpolator as applied to Eq. (8). There are generally two interpolation methods: fixed interpolation and adaptive interpolation. For fixed interpolation, a sync (Nyquist), Gaussian, linear, or a cubic interpolator is applied throughout a channel to estimate the distortion of the channel regardless of channel variation, whereas for adaptive interpolation, for example, a Wiener interpolator using a Wiener filter accurately estimates a channel by adaptively compensating for channel variation utilizing parameters like Doppler frequency and symbol energy per power spectrum density (Es/No).
FIG. 3 is a conceptual view of the fading estimation and compensation using a sync interpolator. As shown in FIG. 3, for channel estimation, a fading estimator 301 estimates fading of pilot symbols and an interpolator 302 interpolates data symbols based on the channel estimation of the pilot symbols. The channel estimation result is reflected in an input signal delayed by a delay 304, to thereby compensate the input signal.
FIGS. 4 and 5 illustrate the other channel estimation scheme, ESAE. A receiver separates an input signal into pilot symbols and data symbols by passing the input signal through a matched filter 401. For the channel estimation, an interpolator 403 is required and data is recovered using the interpolation result. First delay 402 compensates for the signal delay through interpolator 403. Demodulator 405 demodulates the signal. As shown in FIG. 5, recovered data before a symbol xe2x80x9cSxe2x80x9d is used along with pilot symbols xe2x80x9cP1xe2x80x9d, xe2x80x9cP2xe2x80x9d, xe2x80x9cP3xe2x80x9d, and xe2x80x9cP4xe2x80x9d to estimate the channel of the symbol xe2x80x9cSxe2x80x9d.
Despite relative simplicity in channel estimation, the data estimation scheme using pilot symbol channel estimation and the ESAE scheme have shortcomings in that channel estimation is not reliable when a received signal has weak strength or experiences severe fading.
It is, therefore, an object of the present invention to provide a channel estimating apparatus and method capable of channel estimation even in an environment where fading causes severe distortion.
The above object can be achieved by providing a channel estimating apparatus and method in a radio communication system. In the channel estimating apparatus, a fading estimator estimates a channel using preset symbols of an input signal, a first interpolator interpolates the other symbols of the input signal based on the fading estimation, a first inverter inverts the output signal of the first interpolator, a first delay delays the input signal for a predetermined time, a first multiplier primarily compensates the output signal of the first delay by means of the output signal of the first inverter, a second interpolator interpolates each symbol of the input signal relating to primarily compensated symbols in a predetermined period before and after the symbol, a level controller controls the level of the output signal of the second interpolator, a second inverter inverts the output signal of the level controller, a second delay delays the primarily compensated signal for a predetermined time, and a second multiplier secondarily compensates the output signal of the second delay by means of the output signal of the second inverter.